1 - Fungus gnats in aloe vera potted plant.

Yard and Garden April 25, 2009

Q.

Several people in our office have aloe vera plants. These plants have gnats. How can we get rid of the gnats? Would spraying them with soapy water work?

- Karen H.

Albuquerque

A.

The gnats you described are probably fungus gnats. These small, nuisance insects are common in well-watered and over-watered houseplants. The gnat larvae live in the soil and feed on the plants roots and fungi. The adult fungus gnats emerge and fly around creating the nuisance. Treating the larvae is the best way to manage the problem.

There is a pest control product containing Bacillus thuringiensis var. israelensis (a bacteria produced insect toxin) that can be mixed with water and used to drench the soil that is infested with the fungus gnats. It should help. Drenching the soil with insecticidal soap diluted according to the container directions may also help.

Sticky traps (yellow plastic with a sticky substance) will help remove some of the adult fungus gnats from your office environment. Allowing the plants to dry well between irrigations will also help reduce the gnat population. Some plants cannot be allowed to dry enough to solve the problems (aloe vera can be dried enough, so let it dry completely between irrigations). Even with plants that need more water, drying as much as possible without harming the plants will help reduce the problem (not eliminate it). Pots allowed to sit in dishes of water so that the soil at the base is waterlogged will have greater problems. Drain surplus water after watering your houseplants.

The problem with fungus gnats may begin as eggs in new potting soil. Some brands of potting soil seem more likely to have the problem, but a bag of potting soil with a hole in it or after it has been open, may become infested. Try a different brand of potting soil next time you repot and keep the bag tightly closed after opening.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

Links:

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